As is well known in the art, prior to the present invention, single point control of a multi-mode type friction brake system used on each truck portion of a mass transit subway vehicle or on a light rail vehicle has now become a reality in the passenger transit industry. However, with the one force modulation devices now being used and a single control signal which can operate each of the blended service braking, the emergency braking and the wheel slip control, the previously used classic wheel slip safety timer has generally been rendered more or less ineffective as a suitable control device to ensure that the minimum safe brake retardation is being provided by the vehicle braking system. This is especially the situation when conditions are encountered by the transit vehicle such that wheel slip control activity is taking place.
In some of the passenger transit systems in use today, which applicants are aware of, a relatively simple pressure level/timer form of feedback is used as a more reliable control scheme in lieu of the above mentioned safety timers.
One of the more significant disadvantages of these prior art multi-mode friction brake control systems is that they were usually not compatible with all of the different types of braking systems currently being used in the passenger transit industry. The currently used braking systems include, for example, hydraulic, pneumatic or electric brake control systems. Furthermore, the currently used control systems may be, for example, either the digital or analog type.
Furthermore, it is generally well known that many of todays most modern passenger transit systems use an advanced adhesion adaptive wheel slip control system. These advanced control systems normally require that a combination of wheel slip control safety processes be carried out.
For friction brake control arrangements, where a single device is used to provide the blended service brake, the emergency brake and the wheel slip control for its respective truck portion, a safety timer approach has a number of rather serious limitations. In particular, the combination of service/emergency brake control and wheel slip control into a single control device creates rather severe command signal problems, particularly, with respect to the prioritization of these signals. The main reason why this is such a severe problem is because wheel slip control, by its very nature, wants to try to reduce the braking force being applied to a level which is below the commanded levels sufficient to control wheel slippage.
Additionally, when both a separate wheel slip control valve device and a brake control valve device are utilized, the wheel slip control valve device normally wants to remain in a wide open application state when the slippage is not being controlled. On the other hand, the brake control valve device cannot remain in a wide open application state at any time during a normal operation mode. Consequently, the combining of both brake control and wheel slip control into a single operating valve device substantially eliminates simple control command feedback as a viable safety check for brake retardation capability. In fact, it becomes almost impossible task for even the most advanced safety timer processes, available at the present time, to determine whether a control action being taken is either a legitimate brake control action, a legitimate wheel slip control action, or an equipment fault condition that exists.